Viral structure. Viruses are made up of nucleic acid (DNA or RNA) enclosed in a protein coat (the capsid). The individual protein subunits making up the capsid are called capsomeres.

A simplified viral reproductive cycle. A virus is an obligate intracellular parasite that uses the equipment and small precursors of its host cell to reproduce.

The lytic cycle of phage T4, a virulent phage. Phage T4 has about 100 genes, which are transcribed and translated using the host cell′s machinery.

The lytic and lysogenic cycles of phage λ, a temperate phage. After entering the bacterial cell and circularizing, the λ DNA can immediately initiate the production of a large number of progeny phages (lytic cycle) or integrate into the bacterial chromosome (lysogenic cycle).

The reproductive cycle of an enveloped RNA virus. Shown here is a virus with a single–stranded RNA genome that functions as a template for synthesis of mRNA. Some enveloped viruses enter the host cell by fusion of the envelope with the cell′s plasma membrane; others enter by endocytosis.

The structure of HIV, the retrovirus that causes AIDS. The envelope glycoproteins enable the virus to bind to specific receptors on certain white blood cells.

Model for how prions propagate. Prions are misfolded versions of normal brain proteins. When a prion contacts a normal “twin,” it may induce the normal protein to assume the abnormal shape.

Replication of a bacterial chromosome.

Generalized transduction. Phages occasionally carry random pieces of the host chromosome containing bacterial genes from one cell (the donor) to another (the recipient).

Conjugation and recombination in E. coli. The DNA replication that accompanies transfer of an F plasmid or part of an Hfr bacterial chromosome is called rolling circle replication. This is sometimes referred to as the “toilet paper” model.

Regulation of a metabolic pathway. In the pathway for tryptophan synthesis, an abundance of tryptophan can both (a) inhibit the activity of the first enzyme in the pathway (feedback inhibition), a rapid response, and (b) repress expression of the genes for all the enzymes needed for the pathway, a longer–term response. The – symbol stands for inhibition.

Regulation of a metabolic pathway. In the pathway for tryptophan synthesis, an abundance of tryptophan can both (a) inhibit the activity of the first enzyme in the pathway (feedback inhibition), a rapid response, and (b) repress expression of the genes for all the enzymes needed for the pathway, a longer–term response. The – symbol stands for inhibition.

Positive control of the lac operon by catabolite activator protein (CAP). RNA polymerase has high affinity for the lac promoter only when catabolite activator protein (CAP) is bound to a DNA site at the upstream end of the promoter. CAP attaches to its DNA site only when associated with cyclic AMP (cAMP), whose concentration in the cell rises when the glucose concentration falls. Thus, when glucose is present, even if lactose also is available, the cell preferentially catabolizes glucose and does not make the lactose–utilizing enzymes.